I think that's definitely pushy. My vet is pro-spay/neuter as well, but he listened to me when I told him why Dance is not to be spayed and hasn't pushed the issue at all. I'd be pretty annoyed if I got a letter like that in the mail, though I do think it's good for the majority of pet owners. Obviously though you've given valid reasons as to why Blaze will not be neutered, so the clinic should be leaving you alone.